Electron emitter



May' 26, 1936.

J. R. WILSON ET'AL ELECTRON EMITTER Filed June 50, 1933 y wma. mmf,

A 7` TOR/VEY a portion of the material of the carrier or core PatentedMay 26,1936- lUNrrED 4STATES- aoirsoz ELEcraoN Em'rrEa James R.v Wilson,Mountain Lakes, N. J., and John 'l'. Acker, Queens Village, and CharlesD. Hartman, Forest Hills, N. Y., assignors to Bell ,'IjelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication June 30, 1933, Serial No. 678,426 11 claims. (c1. 25o-27.5)

This invention relates to electron emitters and more particularly toelectron' emitting cathodes of the coated type used in thermionicdischarge and space discharge devices. y

The well known dull emitter or oxide coated cathode has been, for a.number of years, produced in two characteristic types, known as theuncombined type and the combined type.

'Ihe uneombined" cathode comprises a metallic carrier or core ofplatinum, platinum alloy, nickel or nickel alloy coated with barium andstrontium compounds of the type which can beeasily decomposed by heat tothe oxides. When this cathode is mounted in an enclosing vessel with theusual cooperating electrodes of an electron discharge device, the vesselevacuated, and the cathode heated, the compounds decompose to the oxidesof barium andv strontium with `no chemical combination with the metallic`carrier or core. This type-of cathode has the white appearance of theoxides of barium and strontium.

The combined cathode comprises a metallic carrier or core of platinum,platinum alloy, nickel or nickel alloy coated with barium and strontiumcompounds of the type which can be easily decomposed by heat to theoxides. The coated carrier or core is heated in air to a hightemperature, approximately 1100 C.,.which causes (1) the barium andstrontium compounds to decompose to the oxides and (2) a portion of thebarium and strontium oxides, the oxygen of the air and to combinechemically to form` complex comgrey or black in appearance. When thistype of cathode is mounted in an 'enclosing vessel with the usualcooperating electrodes of an electron discharge device, the vesselevacuated, and the cathode heated, the complex compounds decompose togive the oxides of barium and'strontium and finely divided corematerial. This cathode retains its dark appearance due to the finelydivided core material dispersed throughout the barium and strontiumoxides. Upon further treatment under appropriate conditions intheevacuated vessel both types of cathodes can be made thermionicallyactive by the formation of small amounts of alkaline. earth metals,notably barium. The thermionic characteristics of these cathodes dependlargely upon` these small amounts of alkaline earth metals inassociation with the matrix of oxides and, in the case of the combined"type, also in association with the finely divided metal from the core.

lo In the case of the "uncombined cathode thermionic activation resultsfrom the ordinary processes of outgassing the cathode and heating theanode and other cooperating electrodes by induced high frequencycurrents. In the case of the combined" cathode thermionic activation is5 attendant upon a bombardment process in which the anode and othercooperating electrodes are maintained at a positive potential in orderto draw space current in a gas at low pressure. The resultingbombardment of electrons and ionized l gas on the electrodes causesheating and outgassing of the parts and simultaneously enhancesthesupply of thermionically active material in the cathode.

Thermlonic devices using these types of cathl odes are subject to lossof thermionic activity with use due to the loss of active material, i.e; barium metal, from the surface of the coating. 'I'he adequacy withwhich this surface is maintained or replenished with active materialfrom a reserve supply in the coating matrix determines -the emissionactivity of the cathode and the length of its useful life.

In this connection it should be noted that the combined cathode has inthe past exhibited a superior characteristic of long life and this facthas been attributed to the presence of finely divided core material inthe oxide coating which acts as'a reservoir for active material. 'I'hecombined" cathode, however, has not been appli. cable to many uses dueto limitations of coating and activation techniques. Thefcombined"cathode technique described above involves the heterogeneous reactionbetween .the core surface, oxygen of the air and the solid barium andstrontium oxides and the subsequent diffusion of the complex compounds,from the surface of the core to within the body of the remaining oxides.This process is also a function or the time and temperature of treatmentand is difiicult to control, hence the amount and state of dispersion offinely divided core material in the coating matrix after activationvaries Within wide limits. Moreover, the composition of the corematerialpredetermines the kind and the amount of finely divided corematerial in the activated cathode, hence the process is limited inapplication. Furthermore the coating technique is not readily applicableto all shapes and sizes of desired cathodes. 'I'he method of activatingthis type of cathode is dim- 5 cult to control, time consuming and.inapplicablel in many cases, notably in vacuum tubes employing amultiplicity of control electrodes and in vacuum tubes employing theequi-potential type of cathode.

With the above ideas in mind, this invention is concerned withanelectrcn emitter of the coatedltype which is not subject to thelimitations of fabrication, activation and application of the combinedtype and which has characteristics of life and thermionic emissionsuperior to those of the present so-called combined and uncombined"types.

An object of this invention is to increase the operating life of oxidecoated electron emitters.

Another object of this invention is to obtain saturation eillciency ofemission from oxide coated emitters with low power consumption.

A further object of this invention is to obtain optimum dispersion ofactive material in the coating matrix of the emitter.

In accordance with this invention any refractory metal suitable for acathode may be used as the carrier or core on which the coating isformed or deposited. A coating mixture is prepared which comprisescompounds of alkaline earth metals and a compound of a metal orcompounds of metals of the eighth group of the periodic system ofchemical elements suspended with a binder in a suitable suspendingmedium. A binder is added to promote the ease of handling the cathodeduring the fabrication of the electron device. The binder decomposes ordisappears during the subsequent heat operations. This mixture is coatedor sprayed onto the carrier or core. The coated carrier or core is thenheated at a low temperature to evaporate the suspending medium and causethe binder to function but not at a suiilciently high temperature tocause chemical combination between the mixture and the carrier or corematerial. The coated carrier or core is then mounted in an enclosingvessel with the usual cooperating electrodes of an electron dischargedevice. The vessel is then evacuated and the cathode heated so that thecompounds are decomposed to oxides. The oxide of the metal of the eighthgroup, which is now finely dispersed throughout the coating, isreducedin situ byA means of a reducing gas whereby the metal is uniformlydispersed throughout the oxides of alkaline earth metals in a definiteamount and in a deilnite state of dispersion. Upon electron bombardmentin carbon monoxide, a portion of the alkaline earth oxides is reduced toalkaline earth metal which diffuses in and through the matrix ofalkaline earth oxides and the free metal of the eighth group where it isdeposited on or alloyed with the constituents of the matrix. It can beseen that by this invention, a definite and reproducible cathode of thecoated type can be produced which has the optimum characteristics ofthermionic activity and long life.

As a specic example of the invention, a triadic composition of bariumand strontium carbonates and nickelous carbonate is mixed with asuitable organic binder, such as nitrocellulose, and a suitable organicsolvent, such es amyl acetate, which gives a suspension of the properviscosity for readily coating or spraying the type of carrier or core onwhich the coating is to be formed. The coated carrier is then heated ata low temperature to remove the solvent. The cathode is then mounted ina vessel` and processed as outlined above to make a thermionicallyactive cathode.

In another specific application of the invention barium and strontiumnitrates are added to the ture to fuse the nitrates. These temperatures.however. are not high enough to cause chemical combination with thecarrier or core material.

In a further specic application of the invention, a mixture of bariumand strontium carbonates and nickelous carbonate is suspended in asolution of Water and barium and strontium nitrates which gives asuspension of the proper viscosity for readily coating or spraying thetype of carrier or core on which the coating is to be formed. The coatedcarrier is then heated at a low temperature to evaporate the water andthen at a higher temperature to fuse the nitrates.

These temperatures, however, are not high enough to cause chemicalcombination with the carrier or core material. The cathode is thenmounted in a vessel and processed as outlined above to make athermionically active cathode.

This invention will be more clearly understood from the followingdetailed description in connection with the accompanying drawing.

Fig. 1 illustrates a coated lament made in accordance with thisinvention.

Fig. 2 is an enlarged view of a portion of the filament shown in Fig. 1with a quarter sector cut away and shown in cross-section to illustratethe complex matrix of this invention.

Fig. 3 illustrates a discharge device of the three electrode typeembodying an emitter made in accordance with this invention and Fig. 4is a diagrammatic arrangement showing the various steps of the coatingand transformation process of one form of the invention.

In accordance with one aspect of this invention, the objects previouslyset forth are accomplished by forming a matrix on the cathode core orsurface i0 which includes alkaline earth oxides H, such asv barium andstrontium, a metal of the eighth group I 2, such as nickel, in a freestate and crystalline form, dispersed throughout the oxides, and freemetallic barium or barium and strontium I3 in the matrix and on thesurface thereof.

The metal I2 distributed through the oxides, primarily is introduced inthe coating as a compound and serves as a depository surface for freebarium in the coating or on the surface thereof and the barium either isalloyed with or is adsorbed on the nickel particles. In effect, the freenickel is covered with a skin layer or iilm of metallic barium. In viewof the large number of particles in the oxide coating and theirdispersed positions between the particles of oxides, it is evi'- dentthat a large amount of free barium is always in reserve in the coatingand this barium is supplied by diifusion to the surface to replace thelm of barium depleted during operation of the emitter.

The adequacy with which this surface is maintained or resupplied from areserve supply in the coating matrix determines the emissive activity ofthe emitter and the length of life or duration of time the emitter maybe considered as emitting a copious supply of electrons. The amount ofsurface barium and the concentration thereof will depend upon and be inequilibrium with the volume concentration of barium and nickel within.the matrix.

Another advantage of the dispersion of nickel throughout the matrix isthe elimination of bright spots in the emitter. This is chieiiy due tothe separation or dispersion of the free nickel in the oxides wherebyfusion of the nickel is avoided.

While nickel has been speciiled as a preferable metal for associationwith the alkaline earth oxide. other metals of the eighth groumsingly orin combinations, may prove equally as good,

such as iron, cobalt," palladium or platinum or? earth metals, and theiroxides, or other ther"-y mionicallyV active substances. y

It should be'realized that when anelectron emitter is mentioned in 'thisspecification it is intended that the emitter maybe employed in avariety of forms depending on the purpose for which it is to be used.For instance an incan descent or directly heated emitter may be thecommon form of a fllainentary conductor. An indirectly heated type maybe an equipotential surface. And a cold emitter or cathode may be ametallic electrode which is rendered active by a suitable potential incombination with aconducting gas or vapor.

In order to secure the advantages of this invention and to realize thelong life of the emitter,

the following description is given to illustrate more clearly howtheinvention may' be practiced.

but it is to be understood that the proportions are merely suggestiveand the invention is not to be bound thereby.

A fllainentary emitter til, such as a fine wire or ribbon core ofnickel, platinum, or alloy thereof is preferably preglowed in a vacuumoven to remove deleterious matter and occluded gases and then subjected'to an annealing temperature treatment in an atmosphere `oi? hydrogen.The treated wire or ribbon may be coated or provided with any number oflayers by dipping or spraying with a suspension of a coating mixturewhich may be prepared as follows:

` i Grams Barium carbonatos 255 strontium carbonate 300 .Barium nitrate50 strontium nitrate 100 Nickelous carbonate 40 The vehicle used forsuch a suspension may be water of the proper proportions to give a wetmixture of the viscosity tosuit` the type of core to be coated. Insteadof water the vehicle may be an organic binder, such' asv cellulosenitrate dissolved in amyl acetate. The mixture should be continuallyagitated during the coating of the filmentary core to secure a uniformdistribution and to prevent settling of the nitrates or carbonates.Nickelous carbonate is recommended because this compound is stable atordinary temperatures, but at the elevated temperatures used indischarge devices, it is easily reduced to the oxide and metal.Furthermore, it is readily `ob tained in pure form and requires noextraordinary precautions in handling. Under proper conditions manyother nickel salts or compounds or compounds or salts ofthe other metalsin the eighth group of the periodicsystem which can be easily reduced toan oxide or metal may be used.

By varying the amount of nickelous carbonate in the original mixture,filaments can be produced having different black body constants therebymaking it possible to produce filaments with any desired powercharacteristic in shapes and sizes hitherto impossible. Intheabovemixture the ratio of nlckelous carbonate to barium and strontiumcarbonatos is substantially 1 to 18.

After the nlament is coated it is heated only to a low temperature todrive od the suspension vehicle. The coated filament it is then heatedin an inert atmosphere up to 675 C. tofuse the barium and strontiumnitrates and form a flux or bond between the core and the coatingmaterial. At this temperature, no chemical combination with the corematerial occurs.' The coated filament it may be mounted in a vessel i5with its cooperating electrodes, such as an anode i6 and grid vi l asshown in Fig. 3. The vessel is then connected to an evacuating systemincluding the usual baking oven enclosing a header which is connected toa mercury aspirator pump backed with a mechanical oil pump.' A liquidair trap may be inserted in the system. The vessel i5 is baked for asuitable length of time to remove water vapor and other gases given offat low tem- Peratures from the vessel after which a suitable lowpressure can 4be obtained in the vessel by means of the vacuum pumps.The emitter is their glowed at the proper temperature in vacuum, todecompose the nickelous carbonate to nickel oxide. The plate 'and gridof the device are heated to a temperature just below the point Whereevaporation of nickel would darken the vessel. This heating may beaccomplished by bombardmentor by heating the plate and vgrid byinducedhigh frequency current. This heat treatment is continued for fiveminutes. During this process the emitter is outgassed by heating to avtemperature whereby the barium and strontium carbonates and nitratesare decomposed to the oxides. After the above transformations haveoccurred in the matrix, and a high degree of vacuum The initialincorporation of the nickel in the form of a compound results in'thefinal product of a finely divided depository for the free barium in thematrix, to serve as a reservoir for replenlshing the bariuni on thesurface of the emitter which is gradually depleted during operation. 'Ihe free nickel in the oxide matrix tends to increase the initialquantity of free barium since the barium is readily alloyed with oradsorbed by the nickel. The free nickel also promotes the retention ofthe barium Within the coating matrix and tends to effect an optimumsurface configuration for the highest thermionic efciency of theemitter.

While the emitter of this invention may be activated by vthe methodheretofore described, other advantageous results may be obtained byactivating the emitter in accordance with the process disclosed in aPatent No. 2,019,504 issued November um and strontium oxides .to producefree barium and strontium in and upon the oxide matrix. In this process,which is known as chemical activation, the free metal is dispersedthroughout the the free nickel which serves as a large depository forthe barium. When the chemical activation process is used in connectionwith the complexl matrix emitter of this invention, the activating gasis removed by further pumping of the device to a low pressure and thenthe device is sealed off the header of the pumping station.

The primary consideration in the production of an emitter of hi'ghthermionic efficiency in accordance with this invention is to obtain anoptimum dispersion of finely divided free metal or metals throughout thecoating matrix, and in fact, the total radiant emissivity of the emittercorresponds roughly to the degree of metal dispersion. In the coatingmatrix of this invention, the ratio of free metal, such as nickel, tobarium and strontium oxides is approximately 1 to 24. This ratio hasbeen found to give the optimum thermionic emission and life. It is ofcourse understood that this ratio may be varied when certain otherfactors become more controlling than optimum emission and life.-

What is claimed is:

1. A coating composition for the production of thermionically activeemitters comprising carbonates of barium and strontium, nickelouscarbonate, and a binder material. i

2. A coating composition for the production of thermionically activeemitters comprising carbonates of barium and strontium, nitrates ofbarium and strontium, a carbonate oi' nickel, and a fluid binder ofnitro-cellulose in amyl acetate.

3. The method of producing a thermionically active emitter whichcomprises coating a core suitable for an emitter with compounds ofalkaline earth metals and a compound of metals of the eighth group,fourth series, of the periodic system, subjecting said core to a heattreatment to decompose the alkaline earth and eighth group compounds tooxides, and reducing the eighth group metal oxide to metal in vacuum.

4. The method of producing thermionically active emitters whichcomprises coating a core suitable for an emitter with compounds ofalkaline earth metals of low and high fusing points and a compound of ametal of the eighth group,

fourth series, subjecting said core to a preliminary heating to fusesaid alkaline earth compounds of low fusing point, heating said core ina vacuum to convert the eighth group metal compound to oxide, heatingsaid core in vacuum to decompose said high fusing point alkaline earthcompounds to oxides, and reducing said eighth group metal oxide to metalwith carbon monoxide in a vacuum.

5. The methodof producing thermionically active emitters which comprisescoating a core suitable for an emitter with nitrates and carbonates ofalkaline earth metals and a carbonate of the eighth group of theperiodic system, subjecting said core to a preliminary heating to fusethe nitrates of alkaline earth metals, heating said core in vacuum todecompose the alkaline earth compounds and eighth group carbonate tooxides, and reducing the eighth group metal oxide to metal with carbonmonoxide in a vacuum.

6. The method of producing a thermionically active emitter whichcomprises coating a core suitable for an emitter with barium, strontiumand nickelous carbonates, decomposing the nickelous carbonate to nickeloxide, decomposing the barium and strontium carbonates to oxides, and

:2,041,802 oxidev matrix and is adsorbed upon or alloyed with y reducingthe nickel oxide to nickel in the presence of carbon monoxide.

7. The method of producing' a thermionically active emitter whichcomprises coating a core suitable for an emitter with barium, strontiumand nickelous carbonates, decomposing the nickelous carbonate to nickeloxide, decomposing the barium and strontium carbonates to oxides,reducing the nickel oxide to free nickel in an atmosphere of carbonmonoxide, reducing barium oxide to metallic barium in the sa'meatmosphere, and associating the metallic barium with the nickel.

8. The method of forming layers on a filamentary core with al coatingsuspension of barium, strontium and nickelous carbonates which comprisesagitating the coating suspension during application to maintain arelative proportion of nickelous carbonate to barium and strontiumcarbonates, dispersing the nickelous carbonate throughout the layers ofbarium and strontium carbonates, decomposing the nickelous carbonate tooxide in the absence of decomposition of the barium and strontiumcarbonates, decomposing the barium and strontium carbonates tooxides,converting the nickel oxide to metallic nickel to the exclusion ofreduction of barium and strontium oxides, and subsequently reducing partof the barium and strontium oxides.

9. The method of coating an electron emitter with a coating suspensionof barium and strontium nitrates and barium, strontium and nickelouscarbonates which comprises agitating the coating suspension duringapplication to maintain a relative proportion of nickelous carbonate tobarium and strontium carbonates, heating the coated emitter `at a lowtemperature in air to fuse the barium and strontium nitrates, heatingthe emitter to convert the nickelous carbonate to nickel oxide, heatingin vacuum to change the barium and strontium carbonates to barium andstrontium oxides, heating the emitter in a reducing atmosphere of carbonmonoxide at a low pressure to reduce the nickel oxide to free nickel,and heating the emitter in the same atmosphere to a higher temperatureto reduce some of the barium oxide to free barium which is associatedwith the free nickel. v

10. The method of forming electron emitters which comprises coating acore with different compounds of alkaline earth metals having high andlow melting points and a compound of a metal of the eighth group of theperiodic system, heating the coated cathode at a low temperature to fusethe low melting point compounds of alkaline earthvmetals, heating thecore in vacuum to a higherl temperature to convert the eighth groupcompound to oxide, heating the core in vacuum to a higher temperature todecompose the high melting point alkaline earth compounds to oxides, andreducing the eighth group oxide to metal with the aid of a reducing gasin vacuum.

1l. A cathode comprising a core metal, an uncombined coating matrixhaving combined characteristics on said core metal, said matrixconsisting of barium and strontium oxides, finely divided particles ofnickel dispersed throughout the oxides, a reservoir supply of freebarium absorbed on said nickel particles, and an atomic layer of bariumon the surface of said matrix.

JAMES R. WILSON. JOHN, T. ACKER. CHARLES D. HARTMAN.

